BUSINESS SEGMENT INFORMATION (disaggregated revenue information) (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($) customers	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) customers	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2020 segment	Dec. 31, 2019 USD ($) segment customers	Dec. 31, 2018 USD ($) customers	Dec. 31, 2017 USD ($) customers
Product and service information [Line Items]
Number of reportable segments | segment										3
Total revenue	$ 522,071	$ 493,593	$ 493,986	$ 499,065	$ 524,677	$ 493,190	$ 488,244	$ 491,914		$ 2,008,715	$ 1,998,025	$ 1,965,556
Forecast [Member]
Product and service information [Line Items]
Number of reportable segments | segment									4
U. S. [Member]
Product and service information [Line Items]
Total revenue										1,888,036	1,871,375	1,859,398
Foreign, primarily Canada and Australia [Member]
Product and service information [Line Items]
Total revenue										120,679	126,650	106,158
Reportable business segments [Member] | Small Business Services [Member]
Product and service information [Line Items]
Total revenue										1,255,779	1,283,620	1,239,739
Reportable business segments [Member] | Small Business Services [Member] | U. S. [Member]
Product and service information [Line Items]
Total revenue										1,157,195	1,180,019	1,150,055
Reportable business segments [Member] | Small Business Services [Member] | Foreign, primarily Canada and Australia [Member]
Product and service information [Line Items]
Total revenue										98,584	103,601	89,684
Reportable business segments [Member] | Financial Services [Member]
Product and service information [Line Items]
Total revenue										633,498	586,967	585,275
Reportable business segments [Member] | Financial Services [Member] | U. S. [Member]
Product and service information [Line Items]
Total revenue										611,403	563,918	568,801
Reportable business segments [Member] | Financial Services [Member] | Foreign, primarily Canada and Australia [Member]
Product and service information [Line Items]
Total revenue										22,095	23,049	16,474
Reportable business segments [Member] | Direct Checks [Member]
Product and service information [Line Items]
Total revenue										119,438	127,438	140,542
Reportable business segments [Member] | Direct Checks [Member] | U. S. [Member]
Product and service information [Line Items]
Total revenue										119,438	127,438	140,542
Reportable business segments [Member] | Direct Checks [Member] | Foreign, primarily Canada and Australia [Member]
Product and service information [Line Items]
Total revenue										0	0	0
Small business marketing solutions [Member]
Product and service information [Line Items]
Total revenue										281,552	292,245	262,192
Small business marketing solutions [Member] | Reportable business segments [Member] | Small Business Services [Member]
Product and service information [Line Items]
Total revenue										281,552	292,245	262,192
Small business marketing solutions [Member] | Reportable business segments [Member] | Financial Services [Member]
Product and service information [Line Items]
Total revenue										0	0	0
Small business marketing solutions [Member] | Reportable business segments [Member] | Direct Checks [Member]
Product and service information [Line Items]
Total revenue										0	0	0
Treasury management solutions [Member]
Product and service information [Line Items]
Total revenue										193,527	148,011	109,240
Treasury management solutions [Member] | Reportable business segments [Member] | Small Business Services [Member]
Product and service information [Line Items]
Total revenue										0	0	0
Treasury management solutions [Member] | Reportable business segments [Member] | Financial Services [Member]
Product and service information [Line Items]
Total revenue										193,527	148,011	109,240
Treasury management solutions [Member] | Reportable business segments [Member] | Direct Checks [Member]
Product and service information [Line Items]
Total revenue										0	0	0
Web services [Member]
Product and service information [Line Items]
Total revenue										166,025	161,646	131,644
Web services [Member] | Reportable business segments [Member] | Small Business Services [Member]
Product and service information [Line Items]
Total revenue										166,025	161,646	131,644
Web services [Member] | Reportable business segments [Member] | Financial Services [Member]
Product and service information [Line Items]
Total revenue										0	0	0
Web services [Member] | Reportable business segments [Member] | Direct Checks [Member]
Product and service information [Line Items]
Total revenue										0	0	0
Data-driven marketing solutions [Member]
Product and service information [Line Items]
Total revenue										157,706	147,893	150,572
Data-driven marketing solutions [Member] | Reportable business segments [Member] | Small Business Services [Member]
Product and service information [Line Items]
Total revenue										0	0	0
Data-driven marketing solutions [Member] | Reportable business segments [Member] | Financial Services [Member]
Product and service information [Line Items]
Total revenue										157,706	147,893	150,572
Data-driven marketing solutions [Member] | Reportable business segments [Member] | Direct Checks [Member]
Product and service information [Line Items]
Total revenue										0	0	0
Fraud, security, risk management and operational services [Member]
Product and service information [Line Items]
Total revenue										87,343	90,105	102,030
Fraud, security, risk management and operational services [Member] | Reportable business segments [Member] | Small Business Services [Member]
Product and service information [Line Items]
Total revenue										24,572	25,460	25,491
Fraud, security, risk management and operational services [Member] | Reportable business segments [Member] | Financial Services [Member]
Product and service information [Line Items]
Total revenue										49,562	50,499	61,185
Fraud, security, risk management and operational services [Member] | Reportable business segments [Member] | Direct Checks [Member]
Product and service information [Line Items]
Total revenue										13,209	14,146	15,354
Marketing solutions and other services [Member]
Product and service information [Line Items]
Total revenue										886,153	839,900	755,678
Marketing solutions and other services [Member] | Reportable business segments [Member] | Small Business Services [Member]
Product and service information [Line Items]
Total revenue										472,149	479,351	419,327
Marketing solutions and other services [Member] | Reportable business segments [Member] | Financial Services [Member]
Product and service information [Line Items]
Total revenue										400,795	346,403	320,997
Marketing solutions and other services [Member] | Reportable business segments [Member] | Direct Checks [Member]
Product and service information [Line Items]
Total revenue										13,209	14,146	15,354
Checks [Member]
Product and service information [Line Items]
Total revenue										783,195	810,389	851,036
Checks [Member] | Reportable business segments [Member] | Small Business Services [Member]
Product and service information [Line Items]
Total revenue										462,950	476,751	482,928
Checks [Member] | Reportable business segments [Member] | Financial Services [Member]
Product and service information [Line Items]
Total revenue										219,695	226,554	249,716
Checks [Member] | Reportable business segments [Member] | Direct Checks [Member]
Product and service information [Line Items]
Total revenue										100,550	107,084	118,392
Forms, accessories and other products [Member]
Product and service information [Line Items]
Total revenue										339,367	347,736	358,842
Forms, accessories and other products [Member] | Reportable business segments [Member] | Small Business Services [Member]
Product and service information [Line Items]
Total revenue										320,680	327,518	337,484
Forms, accessories and other products [Member] | Reportable business segments [Member] | Financial Services [Member]
Product and service information [Line Items]
Total revenue										13,008	14,010	14,562
Forms, accessories and other products [Member] | Reportable business segments [Member] | Direct Checks [Member]
Product and service information [Line Items]
Total revenue										$ 5,679	$ 6,208	$ 6,796
Customer concentration risk [Member]
Product and service information [Line Items]
Number of customers | customers	0				0					0	0	0
Customer concentration risk [Member] | Total revenue benchmark [Member]
Product and service information [Line Items]
Concentration risk, percentage										10.00%	10.00%	10.00%